Catalytic methods for producing lower olefins, such as propylene and isobutylene, by dehydrogenating lower alkanes are known. Various methods include using platinum catalysts, noble metal promoted zinc aluminate spinel catalysts, or chrome-alumina catalysts. However, these catalytic processes suffer from two drawbacks. First, it is difficult to obtain high yields due to equilibrium limitations. Second, the high temperatures typically required for these processes tend to degrade the catalyst.
One type of catalyst commonly used for dehydrogenating lower alkanes is an alumina supported chromia catalyst. Although this catalyst has a relatively high dehydrogenation activity, it suffers from rapid coke formation during the dehydrogenation reaction. Consequently, frequent high temperature regeneration cycles are undesirably required. Due to the need for frequent regeneration, catalysts having a high degree of hydrothermal stability in order to prevent frequent and costly catalyst replacement are desired.
The rapid coke formation and frequent regeneration also necessitate the employment of cyclical processes, such as the Houdry process, when using chromia-alumina as a dehydrogenation catalyst. Cyclical processes make use of parallel reactors that contain a shallow bed of chromia-alumina catalyst. The feed is preheated through a fired heater before passing over the catalyst in the reactors. The hot product is cooled, compressed and sent to the product fractionation and recovery station. To facilitate continuous operation, the reactors are operated in a timed cycle. Each complete cycle typically consists of dehydrogenation, regeneration, reduction, and purge segments. A further requirement for continuous operation is the use of a parallel set of reactors, such as 3 to seven reactors. In an effort to circumvent equilibrium limitations, the reactors are operated at sub-atmospheric pressures during the dehydrogenation cycle (2 to 14 psia). Regeneration is performed with pre-heated air through a direct fire burner or with the exhaust of a gas turbine. Regeneration temperatures range from 550° C. to 700° C.
Because of such severe operating conditions, dehydrogenation catalyst life is typically one to less than two years. Catalyst replacement is performed when conversion and selectivity fall below minimum levels required for the economic operation of the unit. For example, a dehydrogenation catalyst may have an initial conversion and selectivity values of 50–60% and 88–90%, respectively, while end-of-life conversion and selectivity values are typically 40–45% and 75–85%, respectively. Improvements in dehydrogenation catalysts are desired.
Oxygenates constitute a class of gasoline additives. Since passage of the Clean Air Act Amendments of 1990, the demand for oxygenates has been increasing. The most commonly used oxygenates today are methanol, ethanol, and ethers such as methyl tertiary butyl ether (MTBE). Although methanol and ethanol have high blending octanes, problems with toxicity, water miscibility, high Reid Vapor Pressure (RVP), high nitrogen oxide emissions, lower fuel efficiency, and cost make methanol and ethanol less attractive. Consequently, MTBE is particularly attractive. Homologues of MTBE such as ethyl tertiary butyl ether (ETBE) and methyl tertiary amyl ether (TAME) are increasingly attractive.
Ether production capacity is often limited by iso-olefin feedstock availability. In this connection, MTBE and ETBE production processes both utilize isobutylene as a feedstock while TAME production processes utilize isoamylene as a feedstock. Isobutylene and isoamylene are typically supplied to an ether production process from a fluid catalytic cracking unit (FCC), a fluidized or delayed coker, or from downstream paraffin isomerization and dehydrogenation facilities. As a result, the availability of hydrocarbons having 4 or 5 carbon atoms is limited by numerous possible constraints including crude properties, FCC catalyst properties and operating conditions, coking conditions, as well as by other refinery operating constraints. The chemical mix of C4 and C5 paraffins, olefins, and aromatics as well as the particular mix of iso-olefins to normal olefins are similarly constrained.